Rosén, Johanna

Abstract [en]

We have studied oxygen incorporation and defect formation in M2AlC (M = Ti, V, Cr) MAX phases using first principles calculations. Evaluating phase stability and electronic structure for different oxygen and/or vacancy configurations, we show that oxygen prefer different lattice sites depending on M-element, which can be correlated to the number of available non-bonding M d-electrons. The results show that oxygen substitutes for carbon in Ti2AlC, while forming an interstitial oxygen in the Al-layer for Cr2AlC. We also predict that oxygen incorporation in Ti2AlC stabilizes the material, which explains the experimentally observed 12.5 at% oxygen (x = 0.5) in Ti2Al(C1-xOx). Due to similar valence electron configuration of Ti2AlC and the hypothetical M2AlC (M =Zr, Hf), we also investigate if oxygen can be used to stabilize the latter MAX phases.

Abstract [en]

In this thesis, first-principles calculations within density functional theory are presented, with a principal goal to investigate the phase stability of so called Mn+1AXn (MAX) phases. MAX phases are a group of nanolaminated materials comprised of a transition metal (M), a group 12-16 element (A), and carbon or nitrogen (X). They combine ceramic and metallic characteristics, and phase stability studies are motivated by a search for new phases with novel properties, such as magnetism, and for the results to be used as guidance in attempted materials synthesis in the lab.

To investigate phase stability of a hypothetical material, a theoretical approach has been developed, where the essential part is to identify the set of most competing phases relative to the material of interest. This approach advance beyond more traditional evaluation of stability, where the energy of formation of the material is generally calculated relative to its single elements, or to a set of ad hoc chosen competing phases. For phase stability predictions to be reliable, validation of previous experimental work is a requirement prior to investigations of new, still hypothetical, materials. It is found that the predictions from the developed theoretical approach are consistent with experimental observations for a large set of MAX phases. The predictive power is thereafter demonstrated for the new phases Nb2GeC and Mn2GaC, which subsequently have been synthesized as thin films. It should be noted that Mn is used for the first time as sole M-element in a MAX phase. Hence, the theory is successfully used to find new candidates, and to guide experimentalists in their work on novel promising materials. Phase stability is also evaluated for MAX phase alloys. Incorporation of oxygen in different M2AlC phases are studied, and the results show that oxygen prefer different sites depending on M-element, through the number of available non-bonding Md-electrons. The theory also predicts that oxygen substituting for carbon in Ti2AlC stabilizes the material, which explains the experimentally observed 12.5 at% oxygen (x = 0.5) in Ti2Al(C1-xOx).

Magnetism is a recently attained property of MAX phase materials, and a direct result of this Thesis work. We have demonstrated the importance of choice of magnetic spin configuration and electron correlations approximations for theoretical evaluation of the magnetic ground state of Cr2AC (A = Al, Ga, Ge). Furthermore, alloying Cr2AlC with Mn to obtain the first magnetic MAX phase have been theoretically predicted and experimentally verified. Using Mn2GaC as model system, Heisenberg Monte Carlo simulations have been used to explore also noncollinear magnetism, suggesting a large set of possible spin configurations (spin waves and spin spirals) to be further investigated in future theoretical and experimental work.